Field of the Disclosure
This disclosure relates to exhaust gas-driven automotive turbochargers having a rotating turbine wheel. More particularly, this disclosure relates to a pulse energy enhanced turbine having equidistant ports around the face of the turbine wheel for individual pulsating exhaust flow corresponding to each cylinder of the engine.
Description of Related Art
Advantages of turbocharging include increased power output, lower fuel consumption and reduced pollutant emissions. The turbocharging of engines is no longer primarily seen from a high-power performance perspective, but is rather viewed as a means of reducing fuel consumption and environmental pollution on account of lower carbon dioxide (CO2) emissions. Currently, a primary reason for turbocharging is using exhaust gas energy to reduce fuel consumption and emissions. In turbocharged engines, combustion air is pre-compressed before being supplied to the engine. The engine aspirates the same volume of air-fuel mixture as a naturally aspirated engine, but due to the higher pressure, thus higher density, more air and fuel mass is supplied into a combustion chamber in a controlled manner. Consequently, more fuel can be burned, so that the engine's power output increases relative to the speed and swept volume.
In exhaust gas turbocharging, some of the exhaust gas energy, which would normally be wasted, is used to drive a turbine. The turbine includes a turbine wheel that is mounted on a shaft and is rotatably driven by exhaust gas flow. The turbocharger returns some of this normally wasted exhaust gas energy back into the engine, contributing to the engine's efficiency and saving fuel. A compressor, which is driven by the turbine, draws in filtered ambient air, compresses it, and then supplies it to the engine. The compressor includes a compressor impeller that is mounted on the same shaft so that rotation of the turbine wheel causes rotation of the compressor impeller.
Turbochargers typically include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a center bearing housing coupling the turbine and compressor housings together. The turbine housing defines a volute that surrounds the turbine wheel and that receives exhaust gas from the engine. The turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold.
To regulate gas flow, a wastegate turbocharger operates with a wastegate valve assembly (in the turbine housing) which may include a valve, vent and/or bypass that is able to selectively route a portion of the exhaust gas around (i.e. bypassing) the turbine wheel, in order to limit/control turbine work, thus selectively using a fraction of the available exhaust energy that could be extracted from the exhaust gas flow. Thereby, the wastegate valve assembly regulates exhaust gas flow and ensures that the turbine wheel is not spun at an undesirable speed.
There are three primary types of turbines used in turbochargers: axial, radial and mixed-flow. In axial-flow turbines, exhaust gas flow through the turbine wheel is only in the axial direction, along the axis of the turbine wheel. In radial-flow turbines, exhaust gas inflow is centripetal, i.e. in a radial direction from the outside in, and exhaust gas outflow is typically in the axial direction. Initial exhaust gas flow is perpendicular to the axis of rotation. The radial flow turbine wheel has the exhaust gas directed along the radius of the wheel. In mixed-flow turbines, the exhaust gas flow approaches the turbine wheel in a direction between the axial direction and the radial direction.
U.S. Pat. No. 4,850,820 discloses a turbocharger in which gases are directed through the turbine wheel in a generally axial direction. An axial-flow turbine inherently has a lower moment of inertia than does a radial-flow turbine, thus reducing the amount of energy required to accelerate the turbine to operating speed. The low inertia of axial-flow turbines is beneficial for high-speed automotive use, but an axial-flow turbine's low efficiency at diameters matched to the required compressor flow has been a limitation.
An axial-flow turbine typically has lower flow resistance and stress than a radial-flow turbine. Sometimes, axial-flow turbines can be more efficient because the exhaust gas is forced directly against the entire turbine wheel while for radial-flow turbines the exhaust gas flows from the side of the turbine wheel and then around the perimeter of the turbine wheel.
Axial-flow turbines may not perform well at higher expansion ratios, such as are typically needed due to the pulsing nature of the exhaust of an internal combustion engine. Axial-flow turbines also have space limitations for automotive designs.
PCT Application PCT/US2013/26652, filed 19 Feb. 2013, discloses a turbocharger having an axial-flow turbine wheel and an internal heat shield having turning vanes. A heat shield having turning vanes may be used to redirect or guide the exhaust gas to the axial direction before the gas arrives at the turbine wheel.
In multi-cylinder engines, cylinders from opposing banks fire alternately. Exhaust gas flow is not a smooth stream because exhaust gases exit each cylinder based on the engine's firing sequence, resulting in exhaust gas pulses. In the case of a “V” engine, the banks are separated across the engine. In the case of an inline engine, the banks could simply be the front half of cylinders versus the back half of cylinders. The exhaust gas is conducted to turbine housing in separated portions of the volute. The separate gas streams serve to preserve the “pulse” of pressure that occurs when the exhaust gas is released from the cylinder. The preservation of the pulses may be desirable because the extra pulse of pressure can start the turbine moving faster. This can be helpful in reducing turbo lag. In the region where the exhaust gases are admitted to the turbine housing, a separator wall between the two halves of the volute can help preserve the separation between exhaust gases from each cylinder bank, and thus maintain the pressure pulses. A drawback of using an axial-flow turbine wheel and a heat shield in a turning arrangement is that the wheel is now spaced far away from the turbine housing, and hence far away from the housing divider wall and the pulse separation. By adding a divider wall extension to the heat shield, the preservation of pulses can be maintained all the way downstream to the turbine wheel inlet.
In twin-scroll (or two-pulse) turbo systems, divided turbo manifolds have been designed with twin-scroll divided chambers in the volute to enhance the benefits of pulse by separating exhaust gas flow into two branches. Gas flow of an in-line four-cylinder engine from certain cylinders, such as 2 and 3, pass through one branch (passageway) of the manifold, and gas from other cylinders, such as 1 and 4, pass through a separate branch. Gas flow from each branch from respective cylinders stay divided in the twin-scroll in the volute of the turbine housing. The resulting two feed ports (2 and 3 with 1 and 4) deliver opposite and substantially equal firing pulsations to improve turbine efficiency and reduce manifold complexity. Divided manifold runners, such as for in-line four cylinder configurations, enhance pulse utilization by separating exhaust flow with alternating pulses. Similarly, a six-cylinder configuration have cylinders 1, 2 and 3 and combined 4, 5 and 6 as separate branches into two feed ports providing alternating pulses. Twin-scroll turbo systems may have higher backpressure at low rpm (which may help turbo spool-up) and lower backpressure at high rpm (which may help top-end performance).
Thus, it is desired to further improve on separate passageways for preserving individual pulses to the turbine wheel.